1. Field of the Invention
The present invention relates to a dielectric-laminate type band-pass filter for use in a portable radio and the like having frequency of several hundred MHz to several GHz.
2. Description of the Prior Art
It is possible to divide conventional resonators roughly into the resonator using a strip line and the resonator using a coil pattern. In a band-pass filter using such resonators, it is so plural resonators are connected magnetically.
Resonators using a strip line, include a resonator of 1/2 wavelength whose line is open at both ends as shown in FIG. 33, and a resonator of 1/4 wavelength whose line is open at one end and shortcircuited at the other end as shown in FIG. 34.
Meanwhile, as the resonator using the coil pattern, as shown in FIG. 35, there is the one in which a spiral-shaped coil pattern 201 and an earth pattern 203 are formed on both sides of a dielectric layer 202 which is clamped therebetween.
In band-pass filters using the above-mentioned conventional types of resonator, however, the following problems were encountered respectively.
[1] band-pass filter using the strip line
(a) A resonator having the resonance frequency of 2 to 3 GHz becomes substantially large. In particular, in the band-pass filter having a construction in which plural resonators are connected, it becomes considerably large. This is due to the following reasons.
That is, lengths L.sub.10, L.sub.11 of the strip line are determined as shown in Equation 1 (resonator of 1/2 wavelength) and Equation 2 (resonator of 1/4 wavelength). ##EQU1## Where, .lambda.; wavelength, .epsilon.; dielectric constant of dielectric-laminate sheet.
However, at present, a dielectric constant of the dielectric-laminate sheet which is capable of being co-fired with silver or copper and has a good temperature characteristic can not be made so large, only about .epsilon..apprxeq.10. Thus, in the above Equations 1 and 2, when .epsilon.=10, L.sub.10 =15.8 mm and L.sub.11 =7.9 mm, which are very long, thus resulting in a large resonator (band-pass filter) as stated above.
(b) In the band-pass filter, it is desirable to adjust input/output impedances depending on the apparatus into which it is incorporated (for matching of the impedances of the band-pass filter and the apparatus). However, in the case of the strip-line type, since the input and output impedances have specific values for every strip line, it is impossible to adjust the matching, even by changing the input/output position from the strip line.
[2] band-pass filter using the coil-pattern
Since the coil pattern has a spiral shape, magnetic fluxes influence one another between the adjoining patterns, thus an electric current flow is difficult to obtain. Therefore, a substantial resistance increases and the Q value becomes lower.
For example, in FIG. 35, since the electric current flows in a same direction (both in a direction A in FIG. 35) in a pattern piece 201a and a pattern piece 201b, the corresponding magnetic fields cancel each other to cause the magnetic flux to become coarse, and consequently an electric current flow is disturbed and the substantial resistance increases.
A further problem is that an insertion loss of the band-pass filter becomes larger when the Q is reduced.
The passing band frequency of the band-pass filter depends on the resonance frequencies of the resonators. The resonance frequencies of the resonators are determined by the dimensions of their strip lines or coil-patterns, so once these patterns are formed, the resonance frequencies can not be adjusted. Therefore, when the dimensions of the patterns are in error, the passing band frequency of the band-pass filter is shifted from a predetermined desired value, resulting in an inferior product.